The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable to be mounted in a relatively small camera such as a camera to be built in a portable device including a cellular phone and a portable informational terminal, a digital still camera, a security camera, a vehicle onboard camera, and a network camera.
In these years, in place of cellular phones intended mainly for making phone calls, so-called smartphones, i.e., cellular phones with functions of portable information terminals (PDAs) and/or personal computers, have been more widely used. Since the smartphones generally are highly functional as opposed to the cellular phones, it is possible to use images taken by a camera thereof in various applications.
Generally speaking, product groups of cellular phones and smartphones are often composed according to specifications for beginners to advanced users. Among them, an imaging lens to be mounted in the products designed for advanced users is required to have a high-resolution lens configuration so as to be also applicable to a high pixel count imaging element developed in these years.
As a method of attaining a high-resolution imaging lens, there is a method of increasing the number of lenses that compose the imaging lens. However, the increase of the number of lenses easily causes an increase in the size of the imaging lens, so that the lens configuration having a large number of lenses is disadvantageous to be mounted in a small-sized camera such as the above-described cellular phones and smartphones. For this reason, the imaging lens has been developed so as to restrain the number of lenses as small as possible. However, with rapid advancement in these days for achieving a higher pixel count of an imaging element, an imaging lens has been developed so as to attain higher resolution rather than a shorter total track length of the imaging lens. As an example, there is an advent of a camera unit formed to be able to obtain an image that is equivalent to that of a digital still camera by attaching the camera unit to a cellular phone or a smartphone, which is different from a conventional camera unit containing an imaging lens and an imaging element to be mounted inside a cellular phone or a smartphone.
In case of a lens configuration composed of seven lenses, since the number of lenses that compose an imaging lens is large, it is somewhat disadvantageous for downsizing of the imaging lens. However, since there is high flexibility in designing, it has potential of attaining satisfactory correction of aberrations and downsizing in a balanced manner. For example, as a lens configuration composed of seven lenses, an imaging lens described in Patent Reference has been known.
Patent Reference: Japanese Patent Application Publication No. 2012-155223
The imaging lens described in Patent Reference includes a first lens having a biconvex shape; a second lens that is joined to the first lens and has a biconcave shape; a third lens that is negative and has a shape of a meniscus lens directing a convex surface thereof to an object side; a fourth lens that is positive and has a shape of a meniscus lens directing a concave surface thereof to the object side; a fifth lens that is negative and has a convex surface directing to the object side; a sixth lens having a biconvex shape; and a seventh lens having a biconcave shape, arranged in the order from the object side.
According to the imaging lens disclosed in Patent Reference, when restraining a ratio between a focal length of a first lens group composed of the lenses from the first lens to the fourth lens to a focal length of a second lens group composed of the lenses from the fifth lens to the seventh lens in a certain range, it is possible to attain downsizing of the imaging lens and satisfactory correction of aberrations.
The imaging lens described in Patent Reference has a small size, but aberrations on an image plane are not sufficiently corrected and especially distortion is relatively large. Therefore, there is a limit by itself in achieving a high-resolution imaging lens. According to the lens configuration described in Patent Reference, it is difficult to achieve satisfactory aberration correction while downsizing the imaging lens.
Here, such an issue is not a problem specific to the imaging lens to be mounted in cellular phones and smartphones. Rather, it is a common problem even for an imaging lens to be mounted in a relatively small camera such as digital still cameras, portable information terminals, security cameras, vehicle onboard cameras, and network cameras.
In view of the above-described problems in conventional techniques, an object of the present invention is to provide an imaging lens that can attain downsizing thereof and satisfactory aberration correction.
Further objects and advantages of the present invention will be apparent from the following description of the present invention.